In contrast to silicon-based solar cells, which are mainly used, organic thin-film solar cells (thin-film organic photovoltaic cells) have huge potential because of, for example,
(1) low environmental load,
(2) low production cost, and
(3) lightweight and high durability,
and hence have been attracting attention in recent years. Such an organic thin-film solar cell is a photoelectric conversion device composed of organic materials, and, in such an organic thin-film solar cell, an organic semiconductor layer composed of an electron-donating π-conjugated polymer, such as a polythiophene polymer or a polyphenylenevinylene polymer, or an electron-donating low-molecular-weight material, such as a phthalocyanine, and an electron-accepting material such as a fullerene serves as a photoelectric-conversion active layer (photoelectric conversion layer). In particular, bulk heterojunction solar cells in which a nano-composite of an electron-donating material and an electron-accepting material is formed to increase the area of the interface between the materials that contributes to photoelectric conversion (this interface being hereinafter referred to as the DA junction) have been recently actively developed (Patent Literature 1).
The photoelectric-conversion principle of an organic thin-film solar cell is as follows:
(1) an organic semiconductor layer absorbs light to form excitons (pairs of holes and electrons),
(2) the excitons migrate by diffusion to the interface (DA junction) between an electron-donating material and an electron-accepting material,
(3) the excitons separate into charges of positive holes and electrons, and
(4) these charges are transported through the electron-donating material and the electron-accepting material to a positive electrode and a negative electrode to produce electric power. The product of the efficiencies of such steps governs the photoelectric conversion efficiency. The life of excitons is very short and a distance over which excitons can migrate by diffusion is very short: at most several nanometers to less than twenty nanometers.
Accordingly, to increase the photoelectric conversion efficiency, the DA junction of the organic semiconductor layer should be preferably present within a distance that substantially equals to the distance over which excitons can migrate by diffusion; and charge-transport paths along which charges after the charge separation can be rapidly transported to the electrodes are preferably ensured.
Unless the DA junction is distributed within the range of about several tens of nanometers in the organic semiconductor layer, there is a problem that excitons formed in the step (1) are deactivated before reaching the DA junction and charges are not extracted. Unless charge-transport paths of the electron-donating material and the electron-accepting material in the organic semiconductor layer are ensured, there is a problem that charges formed in the step (3) cannot reach the positive electrode or the negative electrode and an electromotive force is not obtained.
In view of the above-described points, to increase the efficiency of a photoelectric conversion device, an object is to form the DA junction within the exciton diffusion range and to ensure charge-transport paths formed of the electron-donating material and the electron-accepting material. Stated another way, an object is to form the network of the electron-donating material and the electron-accepting material in the organic semiconductor layer without isolation (formation of discontinuity) of these materials.
The most typical configuration of an organic photoelectric conversion device is bulk heterojunction in which an electron-donating material that is a π-conjugated polymer such as poly-3-hexylthiophene (hereafter, P3HT) and an electron-accepting material that is a fullerene derivative of [6,6]-phenyl-C61-butyric acid methyl ester (hereafter, PCBM) are mixed and the mixture is formed into a thin film so that DA junction are formed in the entire region of the film.
A photoelectric conversion device formed of an electron-donating π-conjugated polymer and PCBM is advantageous in that production thereof does not require expensive production equipment and can be achieved at a low cost because a film of these organic materials can be readily formed from a solution in which the organic materials are dissolved, by a printing method or a coating method (wet process). However, π-conjugated polymers generally have a problem of having a short life because, for example, they are susceptible to an oxidation reaction with oxygen in the air and have low resistance to strong light radiation. In addition, when such a DA junction formation method is used, there are cases where the networks of electron-donating and electron-accepting materials are not sufficiently formed and transport paths for charge transport are not ensured, which cause a decrease in the conversion efficiency.
In contrast, electron-donating low-molecular-weight materials such as phthalocyanines have high resistance to oxygen and light and are expected to provide solar cells having a long life. However, since the materials have a low molecular weight, it is difficult to form the networks (transport paths necessary for charge transport are less likely to be ensured) and it is also difficult to form films by wet processes. Accordingly, the films have been formed by vapor deposition methods, which incur high cost. However, in recent years, organic thin-film solar cells that contain an electron-donating low-molecular-weight material, and that are produced by a coating method have been proposed (Patent Literature 2), and, in the organic thin-film solar cells, a film is formed by a wet process using a solvent-soluble precursor and the film is heated to be turned into an electron-donating material. For example, photoelectric conversion devices in which an electron-donating material is a benzoporphyrin, an electron-accepting material is a fullerene such as PCBM, and a film thereof can be formed by a coating method are advantageous in that they have higher durability such as oxygen resistance and light resistance than conventional photoelectric conversion devices using electron-donating π-conjugated polymers.
However, in the case of using a solvent-soluble precursor, a heating operation requiring extra time and energy is necessary for heating a film of the precursor to convert the precursor into a functional material for a photoelectric conversion device, which is not necessarily preferable in terms of cost. In addition, as in the cases of using π-conjugated polymers, the network structure of the organic semiconductor layer is “accidentally” formed through phase separation caused by a heating treatment. Thus, isolated electron-donating and electron-accepting materials are indispensably present and hence charge-transport paths are not sufficiently ensured, which causes a decrease in the efficiency.
In recent years, to increase the charge-transport efficiency in the step (4) of the photoelectric-conversion principle, a photoelectric conversion device containing a nanowire-shaped electron-donating material has been proposed (Non Patent Literature 1). Specifically, the shape of P3HT, which is an electron-donating π-conjugated polymer, is controlled to be nanowires, that is, fine wires having a width of about several tens of nanometers to increase the charge-transport efficiency. As a result, the P3HT/PCBM photoelectric conversion device containing P3HT nanowires has a higher photoelectric conversion efficiency than P3HT/PCBM photoelectric conversion devices without containing the nanowires.
Thus, it has been confirmed that photoelectric conversion devices containing polymeric nanowires have enhanced photoelectric conversion efficiency. However, as described above, polymers such as P3HT generally have low durability stemming from low oxygen resistance and low light resistance. Therefore, the problems of photoelectric conversion devices in view of practicality have not been overcome.